Have you ever seen any peculiar shaped rocks like the one in the picture? How do you think these weird lumps of rock come to be sitting up on top of a moorland today? Was it a giant's game of skittles he left behind? Or can geology can give us an answer?
The rock in the picture is called the Pepperpot. It sits up on the North Yorkshire Moors amongst a group of unusual shaped rocks known as The Bridestones. I decided to go and have a closer look and see if there were any clues as to how these rocks got there.
After a frosty walk from my village I arrived at the Pepperpot. First I went up close to investigate what kind of rock the Pepperpot was made of. When I ran my hand over the surface it felt quite rough and bumpy and reminded me of sandpaper. As I looked closer I could see that the rock was made of lots of grains of sand. Now you and I know that normally we find sand at the beach, or in a desert, but not up on top of a moor. How did this sand get here?
It turns out that these sandstones were laid down in the Jurassic period, about 180 Million Years ago. At this time the dinosaurs were still stomping around, the climate was a lot more tropical and North Yorkshire was lying under a shallow sea. The sand that makes up the Bridestones today was settling onto the sea floor at this time. Over the years the sand was compressed until it became cemented together into a sandstone.
Another thing I noticed was that there were very fine layers, only millimetres thick in the sand. These layers were created when the sand was laid down under the sea in underwater sand dunes. Geologists call these fine layers cross bedding. Next time you go to the beach have a look at the shapes the sand makes on the beach. Can you imagine what it might look like many years from now when it is compressed into a rock?
After looking at the pepperpot from close up I decided to step back a few paces to see what else I could see. What I noticed was some very large layers in the rock that made it look like an interesting sandwich. The biggest layers were as tall as me while the smallest layers were about the length of a finger. I also saw cracks that ran up and down the rock.
The large layers are called bedding planes and are created while the sand is being laid down underwater. Firstly some sand is laid down in underwater dunes. Next, perhaps there is a storm and the tops of the sand dunes are eroded. Then there is another calm period and more underwater dunes are laid down on top of the old cut off dunes. This happens many times and creates layer upon layer of cut off sand dunes.
Much later, when the sand had begun to cement into a sandstone, the vertical cracks were created. They are known as joints and form when the rock is squeezed by other rocks pressing from either side.
When the sea level dropped, the sun, rain, wind, snow and ice battered at the freshly emerged sandstones. The bedding planes were weak and eroded easily giving us the sandwich like layers we see on the Bridestones.
One last question I had was why are these rocks often mushroom shaped with a very thin bit at the bottom? To answer this I had to go home and have a look in a book.
The extra weathering at the bottom is caused by the wind but with an added twist. After the last of the glaciers left North Yorkshire the land had very few plants growing there. This meant that strong winds would pick up sand grains and bounce them along the ground. These sand grains would crash into the bottom of the rocks and weather any soft layers leaving a mushroom shaped rock.
The Bridestones aren't the only rocks with such unusual shapes and you may have seen similar rocks in other areas of Britain. Next time you see a strange shaped rock just think what an interesting story of might have to tell!
Copyright Kate Ravilious